The colon carcinogen 2', 3-dimethyl-4-biphenylamine and the hepatocarcinogen 2-aminofluorene have significant chemical similarities. Their different biological specificities pose a significant question with respect to their modes of chemical carcinogenesis. Information on their modes of action is being sought by examining and comparing their metabolism, especially with regard to nitrogen modifications. Metabolites of the parent compounds will be determined by bile. Modifications caused by intestinal bacteria will be determined. High pressure liquid chromatography has been the most effective approach.